The present invention relates generally to negative ion generators, and more particularly to a negative ion generator utilizing a magnetically insulated transmission line.
The magnetic confinement fusion program is currently searching for good techniques to heat magnetically confined plasmas to the 10 keV temperatures required for fusion. Ohmic heating is only sufficient to reach temperatures of a few keV because of the T.sup.-3/2 scaling of plasma resistivity. The leading candidate for bridging the gap between temperatures achievable by Ohmic heating and 10 keV has been neutral beam heating. This heating method uses energetic neutrals, usually 60-120 keV hydrogen or deuterium, to deposit energy in the plasma. The energy deposition length in the plasma scales with neutral beam energy. Future generations of tokamaks which simulate reactor conditions will have dimensions and densities so large that currently available neutral beams will only produce skin heating. The 300 to 2000 keV neutral beam energies that are necessary to heat these machines efficiently cannot be achieved using the present technique of converting positive ion beams to neutral beams in charge exchange cells, because the efficiency of these cells decreases exponentially with increasing beam energy. The efficiency of present neutral beam systems is already marginal at 170 keV.
It has been previously recognized that a possible solution to this problems lies in the production of negative ion beams, as these beams can be neutralized with high efficiency.
L. R. Grisham, et. al., suggest in "Plasma Heating with Multi-MeV Neutral Impurity Beams," Princeton Plasma Physics Lab. Report PPPL-1759, March 1981, the use of negative ions with atomic weights of 6 to 20, such as lithium, carbon and oxygen, at energies of up to 1 MeV per AMU. However, the highest output achieved by large area negative ion sources is only one ampere at 0.01 amp per centimeter square at a beam energy of 40 keV. Small area discharge sources have produced intensities of several amps per centimeter square, but only with total current less than 1 ampere and at beam energies less than 60 keV.